The present invention relates, in general, to steam generators which combust fuels and which are used for the production of steam for industrial uses or electric power generation. In particular, the present relation relates to a low NOx Cyclone furnace steam generator and a method of operating same which employs in various combinations redesigned or modified Cyclone furnaces, new and/or relocated air ports, and different operating methods for these Cyclone furnaces and the steam generator. These features can maintain and/or reduce NOx emission levels from the steam generator while minimizing corrosion problems which might otherwise lead to major Cyclone furnace and steam generator tube wastage.
Cyclone furnaces were developed by The Babcock & Wilcox Company (B&W) in the USA in the 1940's. These Cyclone furnaces had the ability to burn high-ash low-fusion temperature coals, which are particularly troublesome in pulverized coal boilers. The Cyclone furnace utilizes centrifugal forces to suspend burning fuel particles, according to their size, in equilibrium against the drag of inwardly directed air flow. The Cyclone furnace has been used with various boiler types manufactured by The Babcock & Wilcox Company including: Stirling (SPB), Radiant Boiler (RB) and Universal Pressure (UP®) boilers, the latter including both subcritical and supercritical designs.
The Cyclone furnace (as schematically shown in FIG. 1) consists of a Cyclone burner connected to a horizontal water-cooled cylinder, the Cyclone barrel. Air and crushed coal are introduced through the Cyclone burner into the Cyclone barrel. The larger coal particles are thrust out to the barrel walls by the cyclonic motion of combustion air where they are captured and burned in the molten slag layer that is formed; the finer particles burn in suspension. The mineral matter melts and exits the Cyclone furnace via a tap at the Cyclone re-entrant throat that leads to the floor and then to a water-filled slag tank (not shown). The combustion gases and remaining ash leave the Cyclone furnace and enter the main combustion chamber. For additional details and a general description of the characteristics of such Cyclone furnaces, the reader is referred to the following sources of information: Chapter 14 of Steam/its Generation and Use, 40th Edition, Stultz and Kitto, Eds., © 1992 The Babcock & Wilcox Company; Chapter 10 of Steam/its Generation and Use, 39th Edition, © 1978 The Babcock & Wilcox Company; Chapter 10 of Steam/its Generation and Use, 38th Edition, © 1972 The Babcock & Wilcox Company; and Chapter 28 of Steam/its Generation and Use, 37th Edition, © 1963 The Babcock & Wilcox Company, the texts of which are hereby incorporated by reference as though fully set forth herein.
Air staging techniques have been used to reduce the production of nitrogen oxides or NOx from steam generators employing such Cyclone furnaces. As described in a technical paper titled “B&W's Advances on Cyclone NOx Control Via Fuel and Air Staging Technologies” by Farzan et al., presented to the EPRI-DOE-EPA Combined Utility Air Pollutant Control Symposium, Aug. 16-20, 1999 in Augusta, Ga., the basic theory of air staging is to reduce the fuel NOx component within the burner zone by reducing oxygen availability. Additionally, the thermal NOx component will be lowered because of the lower combustion gas temperatures. This latter aspect is significant for steam generators equipped with Cyclone furnaces because the thermal NOx component is typically higher than that obtained with other firing techniques. Finally, due to the inherent combustion characteristics of Cyclone-equipped furnaces during staged operation, specific gas species are formed in the lower furnace to promote reburning technology NOx reduction reactions to occur. Cyclone furnace air staging involves reducing the combustion air provided to the Cyclone furnaces and operating the Cyclone furnaces at reduced stoichiometries, typically at a stoichiometry of about 0.90 to 1.00 (less air than is theoretically required for complete combustion). The balance of the theoretical air required for complete combustion, as well as the excess air normally supplied in such combustion processes, is introduced into the main combustion chamber of the steam generator via overfire air (OFA) ports located at a higher elevation than the Cyclone furnaces. As a result, Cyclone air staging employs multiple combustion zones within the main combustion chamber of the steam generator, defined as the main combustion (Cyclone region) and burnout (OFA ports to the furnace exit) zones.
As described in the aforementioned STEAM references, the Cyclone furnaces themselves operate at extremely high heat release rates and with a molten sticky slag layer over a refractory lining which both protects the water-cooled walls of the Cyclone barrel and minimizes heat absorption, thereby assisting in this molten slag remaining in that state so that it can continuously flow out the slag tap into the slag tank (not shown). Thus, Cyclone furnace operation is a unique combustion process. While air staging technology has been successfully used to reduce NOx emission levels from Cyclone fired steam generators, operation in this manner does create potentially negative effects, including higher unburned carbon levels, increased boiler fly ash percentages (higher particulate loading), increased steam generator slagging/fouling, increased corrosion and higher opacity levels. Of particular concern is the potential for reduced Cyclone furnace and lower/upper steam generator main combustion region life expectancy due to corrosion. Unfortunately, with staged combustion, the reduced amount of air supplied to the Cyclone furnaces creates a reducing atmosphere in the lower furnace that leads to furnace wall and floor wastage. The furnace walls of the lower furnace are routinely protected with studs and refractory coating. Refractory alone cannot always be maintained to address these problems. Depending on the individual Cyclone furnace and steam generator design operating temperature and pressure, these problems can be greatly magnified, especially when applying the air staging technology to reduce NOx emission levels.
To minimize the overall furnace wastage issue, some operators of steam generators employing Cyclone furnaces have increased the air flow to the Cyclone furnace to help minimize these concerns, but unfortunately, this mode of operation also increases the resulting NOx emission levels. Dealing with the increased NOx emission levels could require more costly alternative NOx control measures, and could involve the provision of additional NOx removal equipment downstream. To deal directly with the increased corrosion potential within the Cyclone furnaces and the main combustion region of the steam generator, higher cost alternative furnace wall stud and tube materials and/or refractory and/or protective coatings could be employed. However, it is apparent that no proven approach has been provided that can maintain or even lower the NOx emissions from such Cyclone fired steam generators while minimizing these other related negative consequences of furnace tube wastage.